Oil & Gas Science and Technology – Rev. IFP, Vol. 58 (2003), No. 2, pp. 299-311
Copyright © 2003, Éditions Technip
Gas Isotopes Tracing: an Important Tool
for Hydrocarbons Exploration
A. Prinzhofer1 and A. Battani1
1 Institut français du pétrole, 1 et 4, avenue de Bois-Préau, 92852 Rueil-Malmaison Cedex - France
e-mail: [email protected] - [email protected]
Résumé — Les isotopes du gaz utilisés comme traceurs : un outil important pour l’exploration des
hydrocarbures — Le gaz naturel peut être considéré aujourd’hui à la fois comme un traceur fiable pour
la connaissance des hydrocarbures liquides qui peuvent lui être associés, et comme un enjeu économique
en lui même. La mesure isotopique combinée des isotopes stables et des gaz rares donne des contraintes
majeures pour reconstruire l’histoire géologique des hydrocarbures, depuis leur genèse jusqu’à leur
accumulation. Des progrès récents dans la mesure des isotopes stables du carbone du gaz naturel (du
méthane au butane et du dioxyde de carbone), liés principalement à l’utilisation de GC-C-IRMS (Gas
Chromatograph-Combustion-Isotopic ratios mass spectrometer) ont permis de retracer de nombreux
processus physico-chimiques qui affectent le gaz naturel, au lieu d’utiliser ces signatures comme de
simples empreintes digitales d’origines, comme c’était l’usage auparavant. Ces reconstructions fournissent
des informations capitales à la fois sur leur origine et sur l’évolution dynamique des fluides hydrocarbures
entre la roche mère et l’accumulation dans les réservoirs. L’association de cette méthodologie avec
l’utilisation d’autre traceurs naturels augmente notre connaissance de l’histoire des hydrocarbures dans les
bassins sédimentaires. Entre autres méthodologies potentielles, les isotopes des gaz rares peuvent
apparaître comme l’ultime outil de diagnostic, leur totale inertie chimique nous permettant de les utiliser
tant comme des traceurs fiables de sources, que comme des indicateurs quantitatifs de processus
physiques (état des phases, migration et fuites au travers des formations géologiques). Qui plus est,
comme certains isotopes (4He, 40Ar par exemple) sont produit par radioactivité naturelle, il représentent
des horloges géologiques, donnant accès à la quantification des temps de résidence des hydrocarbures
dans les réservoirs. Plusieurs applications nouvelles sont présentées dans cet article, associant les isotopes
stables avec ceux des gaz rares : une nouvelle méthode permettant de distinguer l’origine bactérienne
ou thermogénique du gaz, et la caractérisation de paramètres en relation avec la genèse du gaz
thermogénique (craquage primaire ou craquage secondaire, degré d’ouverture du système, relations entre
les signatures isotopiques et la biodégradation). Enfin, nous présentons une tentative de quantification de
la proportion d’hydrocarbures ayant fui hors d’un réservoir, et des valeurs relatives des temps de résidence
des hydrocarbures dans un système de réservoirs hétérogènes.
Abstract — Gas Isotopes Tracing: an Important Tool for Hydrocarbons Exploration — Gas may be
considered today both as a reliable tracer for the understanding of associated liquid hydrocarbons and
as an economic target. Isotopic measurements of stables isotopes and noble gases give important clues to
reconstruct the geological history of hydrocarbons from their generation to their accumulation. Recent
analytical advances in carbon isotopes of natural gases (methane to butane and carbon dioxide) due
mainly to the development of GC-C-IRMS (Gas Chromatograph-Combustion-Isotopic ratios mass
spectrometer) have allowed to reconstruct some of the physico-chemical processes which affect natural
gas chemical signatures, instead of using these signatures as simple fingerprinting of origins as it was the
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case some time ago. These reconstructions provide important information on both the origins and the
dynamic behavior of hydrocarbon fluids between the source rocks and the accumulations in reservoirs.
Correlating this methodology with other natural tracers increases the understanding of hydrocarbon
history in sedimentary basins. Among other new potential methodologies, noble gas coupled to stable
isotopes are the new frontier tool, as their chemical inertia enables their use as precise tracers of sources
and of associated physical processes (state of the phases, migration and leakage). Moreover, because
some isotopes (4He, 40Ar for example) are produced by natural radioactivity, they may very well
represent geological “clocks”, giving potentially a quantification of the residence times of hydrocarbons
in a reservoir. Several new applications of this association of stable isotopes and noble gases are
presented in this paper: a new way for distinguishing bacterial and thermogenic gas origins, and the
characterization of the parameters related to the genesis of thermogenic gas (primary versus secondary
cracking, openness of the system, relations between gas isotope signatures and biodegradation). At last is
presented a tentative quantification of the proportion of hydrocarbon gases leaked from a reservoir, and
a quantitative relative residence times of hydrocarbons in heterogeneous reservoirs.
INTRODUCTION
The exploration of natural gas became during the last decade
as important as the liquid hydrocarbons quest. Although a
large part of the R&D work performed for petroleum
exploration could be used for the understanding of gas
accumulations, it rapidly appeared that some specific
problems were to be addressed specifically for the
understanding of gas history. Therefore, some geochemical
tools could be especially valuable for gas characterization.
Natural gas is a relatively simple mixture when compared to
oil, as it contains fewer gas molecules in major proportions:
the hydrocarbon part consists of methane, ethane, propane,
normal-butane and isobutane (the isomers of pentane are
already highly fractionated between the liquid and gas phase
at a separator, and cannot be generally used as reliable data).
The three most common nonhydrocarbon gases are CO2, N2
and H2S. Their concentration ranges from 0 to almost 100%,
and they are considered a hazard in gas exploration. Among
the other molecules in trace amounts, the whole family of
noble gases (He, Ne, Ar, Kr, Xe and Rn) is always present in
natural gas, and other compounds as hydrogen or mercury
have been sometimes detected. Apart from the chemical
proportions of these different molecules, the use of isotopic
ratios has been rightfully adapted to oil and gas
interpretations. Part of the reason is that of the GC-C-IRMS
technique (coupling of a gas chromatograph with a mass
spectrometer devoted to stable isotopes measurements)
changed drastically the amount and quality of data obtainable
from gas samples. Ten years ago, the whole analysis of
carbon isotopes of one gas sample took several days, whereas
it is now possible on a routine basis in half an hour.
Moreover, this online technique allows to analyze much
smaller volumes, rendering possible today the analysis of the
carbon isotopic signature of an hydrocarbon molecule with
an abundance as low as 100 ppm, which was not feasible
before. Likewise, very recent technological development of
the same kind of online analysis for the hydrogen isotope
composition of hydrocarbons will probably give a new
geochemical leap for the study of hydrocarbon gases in the
next decade. The implementation of noble gases and
respective isotope studies are still in a process of new
development and research for the purpose of hydrocarbon
exploration. However, the wide knowledge of the geochemical behavior of these natural tracers, associated with
some pioneer work in oil and gas characterization (Zartmann
et al., 1961; Bosch and Mazor, 1988; Ballentine and
O’Nions, 1994; Ballentine and Sherwood-Lollar, 2002)
indicates that their generalized use is probably of paramount
importance for a better understanding of petroleum system.
The reconstruction of the history of a petroleum system
involves several steps. Some of them are already well
developed with the association of organic geochemistry and
basin modeling. However, several other aspects still need a
better quantitative estimate: reservoir continuity, distance and
rate of migration of the hydrocarbons from the source rocks
to the reservoir, and from the petroleum system to the
atmosphere, age and residence time of accumulated
hydrocarbons, origins and sources of natural gas, prediction
of the risk of nonhydrocarbon gases as CO2 and N2. A more
accurate attention given to the chemical and isotopic
signatures of natural gases (hydrocarbons and nonhydrocarbons) may have other industrial implications, for
example in the field of CO2 sequestration, or for the
estimation of potential productions of helium as a by-product
of gas accumulations. We present in this paper some
examples of these new methodologies for several case
studies, highlighting the performances for the prediction of
quality/volume during exploration, by using stable isotopes
associated with noble gas isotopes.
Stable isotopes are natural tracers related to the source
signature of gas samples, and fractionated by the chemical
and physical processes which affect in a slightly different
way the different isotopes of a same chemical compound,
with the general rule that lighter isotopes present a higher
chemical reactivity, and are more mobile than the heavier
A Prinzhofer and A Battani / Gas Isotopes Tracing: an Important Tool for Hydrocarbons Exploration
ones. Their actual isotopic signature is therefore a
superposition of a lot of chemical and physical informations,
rendering sometimes difficult their interpretation.
On the other hand, noble gas molecules have the unique
property to be absolutely inert chemically, and to allow the
tracing of sources and of physical processes of segregation
during the history of a petroleum system without any
interference with chemical processes (Ballentine and
O’Nions, 1994). The second important advantage of noble
gases is that they may be classified in two main groups.
Some of them are fossil, created during the formation of the
solar system, with their main reservoir being the atmosphere,
and completely conservative as they are not chemically
reactive. These molecules enter hydrocarbon fluids through
their solubilization in water in the zones of water recharge,
and are then exchanged and fractionated between crustal
fluids (gas, oil and water). The other group comprises species
produced through natural nuclear reactions, and may
accumulate through time, offering potential natural clocks
inside the crustal fluids. More in-depth will be discussed in
Chapter 4 about radiogenic/nucleogenic production. Interestingly, their large range of molecular mass (from 3 for
helium to 136 for xenon) gives a large spectrum of physical
properties (solubilities between phases, diffusion coefficients,
etc.). These various properties, well constrained through
numerous academic studies, may and are potentially used
quantitatively for petroleum system evaluation.
Among the numerous applications which are being
developed associating carbon isotopes and noble gas
geochemistry, one may quote:
– The source of nonhydrocarbon gases like CO2 and N2.
Indeed, the stable isotopes of these two compounds
generally do not give a unique answer for their origins,
whereas the isotopic signature of helium 3He/4He indicates
the possible proportion of these molecules coming from
the mantle, versus those accumulated directly from a
continental crust (Battani, 1999; Battani et al., 2000).
– The distinction between a bacterial and a thermogenic
origin of natural gas may be confirmed by a completely
novel methodology using the apparent production ratio of
two radiogenic isotopes of noble gases, 4He and 40Ar.
– Segregation during the migration of natural gas may be
monitored through the carbon isotopes of methane
(Prinzhofer and Pernaton, 1997; Prinzhofer et al., 2000a).
However, these signatures are strongly dependent upon the
genesis of the gas (bacterial versus thermogenic) and on
the maturity of the source, obscuring the part of the
fractionation due to gas migration through porous rocks.
On the contrary, because of their inertia, isotopic and
chemical ratios of noble gases are fractionated only by
physical processes, i.e. in this case the interaction between
fluids and fractionation during diffusion. The projection of
such data in migration maps at the basin scale or at the
301
reservoir scale are coherent with stable isotopes but also
much less “noisy”.
– As the fossil isotopes of noble gases enter the gas phase
from the water phase, their absolute and relative
concentrations are directly linked to the amount of water
equilibrated with the gas accumulation. This direct
estimate of the importance of water contact with
hydrocarbons has obvious direct implications for any
estimation of a risk of biodegradation or hydrodynamism
in a petroleum reservoir.
– The amount of radiogenic noble gases is linked to several
key parameters, the amount of parent isotope, the
efficiency of accumulation in the hydrocarbon phase and
the time. This last parameter is extremely important, as it
allows to test for the first time direct chronological
information for hydrocarbons accumulations.
– Finally, apart from the application to petroleum exploration,
and because of their chemical inertia, noble gases may be
used to monitor multi-layered or compartmentalized fields
production, the relative proportions produced from each
block of the field versus time. This kind of new
geochemical monitoring of a multi-play gas field may give
a precise and less expensive information about the
evolution of the production.
We shall now provide more details of some examples of
these different applications.
1 BACTERIAL METHANOGENESIS VERSUS THERMAL
CRACKING: CHARACTERIZATION OF GAS
ORIGINS AND NEW AMBIGUITIES
The origins of methanogenesis in hydrocarbon reservoirs
have been extensively studied through the last decades
(Colombo et al., 1970; Stahl, 1977; Bernard et al., 1977;
Schoell, 1980, 1983; Galimov, 1988; Faber et al., 1992;
Prinzhofer and Huc, 1995; Prinzhofer et al., 2000a;
Prinzhofer et al., 2000b). Even if some unconventional
methane generation has been noted to occur through
marginal inorganic processes (Charlou and Donval, 1993;
Charlou et al., 1998; Guo et al., 1997; Wang et al., 1997;
Szatmari, 1989), the main origins of hydrocarbons are
organic. They are traditionally subdivided in two classes of
processes: one process occurs during the thermal degradation
of organic matter, and is associated with the generation of
large amounts of heavier gas molecules and liquid
hydrocarbons; the other generation process occurs through
bacterial methanogenesis with insignificant amounts of other
hydrocarbon molecules. The distinction between these two
genetic processes is still subject to discussion in several case
studies. Bacterial contamination is an important issue when
characterizing the gas origin and migration with stable
isotopes, as the correct interpretation of the data is sometimes
delicate: in the last two decades, it was deemed easy to
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100 000
10 000
Bacterial
1000
C1/(C2 + C3)
distinguish gas generated by thermal cracking from
methanogenesis using the carbon and hydrogen isotopes
(Bernard et al., 1977; Schoell, 1983: Faber et al., 1992).
These authors demonstrated that bacterial genesis of gas gave
gas enriched in methane, isotopically light compared to a
thermal generation. Several diagrams were proposed to
illustrate the distinction between a bacterial field of
generation and a thermogenic one (in Figure 1 is represented
the first of these diagrams, from Bernard et al., 1977). Gases
with intermediate compositions were interpreted by these
authors as mixes between a bacterial and a thermogenic endmember. The supplementary information obtained with the
hydrogen isotopic ratio of methane enables to add a
characterization of the biogenic path which occurred, i.e. the
fermentation of acetates or the reduction of CO2 (Schoell,
1983; Faber et al., 1992).
However, recent studies have shown that bacterial
oxidation of bacterial gas may give a residual gas showing a
thermogenic signature, whereas diffusive processes during
migration may give a bacterial signature to a thermogenic gas
(Prinzhofer and Pernaton, 1997). In Figure 2 are plotted the
results of several migration experiments (where the gas
migration occurs by diffusion, through a slice of shale or
sandstone, saturated or not with water) where it is possible to
obtain a purely bacterial signature for methane, or a signature
supposedly corresponding to a mixing, when the gas is
actually a purely thermogenic gas. This indicates that
100
Thermogenic
10
1
-70
-60
-50
-40
-30
-20
δ13C1
Figure 1
Distinction of the two different organic geneses of natural gas
(bacterial versus thermogenic). This distinction is based on
the dryness of bacterial gas, and on the fact that the carbon
isotopic ratio of methane is lighter for bacterial gas than for
thermogenic gas. From Bernard et al. (1977), modified.
Figure 1 represents the genetic signature of the gases, but that
post-genetic processes as chemical and bacterial alteration, or
physical migration may change drastically these original
fingerprints.
Diffused methane
δD1
-150
Gas/Gas
-200
Gas/Water
Bacterial
Source
-250
Thermogenic
Mixing
-75
-65
-45
-55
δ13C
-35
1
Figure 2
Experimental results of methane migration through shales, saturated or not with water. The migration occurs by water solubilization/
diffusion, and the data are presented in a hydrogen versus carbon isotopic ratios. The ranges of genetic signatures corresponding to bacterial
and thermogenic methane are also represented (from Faber et al., 1992), showing that a purely thermogenic methane may present a bacterial
signature (or a mixed one), due to its migration through a porous rock. The series of data correspond to a migration through water saturated
shales, but with different petrophysical properties. From Prinzhofer and Pernaton (1997), modified.
A Prinzhofer and A Battani / Gas Isotopes Tracing: an Important Tool for Hydrocarbons Exploration
Apulia (Italy)
Reservoired gas
C2/C1
0.08
303
North Sea
Headspace gas
C2/C1
0.3
0.06
0.2
xin
g
Mi
xin
g
0.04
Mi
0.1
0.02
Diffusion
0
a)
0
-60
-70
-40
δ13C1
b)
-50
δ13C1
-30
Figure 3
Two examples of gas series in the mixing diagram C2/C1 versus δ13C of methane. a) gas data from Apulia (Ricchiuto and Schoell, 1988),
showing a straight line interpreted as a mixture between a thermogenic and a bacterial end-member. b) gases from headspace gases collected
during a drilling of a North Sea field. The curved trend is incompatible with a mixing between two end-members, and is interpreted as the
result of a chemical and isotopic fractionation during the leakage of a thermogenic fluid accumulated in a reservoir located below. From
Prinzhofer and Pernaton (1997), modified.
In some cases, it is then possible to distinguish these
processes using the principle of the mixing diagrams.
Prinzhofer and Pernaton (1997) nonetheless demonstrated
that for gases with isotopically light methane (supposedly
contaminated by bacterial activity), a mixing process could
not explain the observed geochemical trends, and that a
fractionation linked to post-genetic processes as migration
and adsorption/desorption needed to be invoked. Furthermore, in some other case studies, even with mixing diagrams,
the obtained results remained ambiguous. Figure 3 shows
data from Apulia (Italy), which are consistent with a mixing
scenario between bacterial and thermogenic end-members,
and data from headspace gases incompatible with a mixing,
and interpreted as due to a fractionation during leakage of the
gas out of the reservoir.
The authors have developed very recently a totally
independent way of distinguishing these two gas origins
(Prinzhofer, 2001), using the ratios of two radiogenic noble
gases, 4He and 40Ar* (radiogenic part of total 40Ar), as
they are not affected by chemical secondary processes.
Additionnally, physical processes should not fractionate
helium and argon in the range of the observed variations. As
these molecules are generated from the mineral network,
their ratio in a gas phase depends only on the production ratio
in the rocks and the physical fractionation during their
transfer from the solid to the gas phase. As helium diffuses
more easily than argon, the ratio 4He/40Ar* is expected
higher in bacterial gas, as the generation temperature is low
enough to forbid the major part of argon to leave the solid
(Elliot et al., 1993). On the contrary, the higher generation
temperatures of thermogenic gas promote a very effective
transfer of both molecules, and the 4He/40Ar* ratio measured
in these gases is always close to the average crustal
production ratio. Several well-documented natural examples,
with unambiguous origins of the gases, validate this new way
of characterizing gas generation (Fig. 4). However, it
becomes clear with such examples that the gap supposedly
existing between the bacterial generation and the thermal one
diminishes. The estimation of newly discovered hidden
biomass, the possibility for some bacteria to live in extreme
thermal conditions pushes the limits of bacterial
methanogenesis probably in the range of the beginning of the
thermal degradation of organic matter. The ancient
observation of an “early thermogenic gas” which would be
dry and non associated with oils may be reconciled with new
observations concerning the behavior of reduced carbon
cycles in a sedimentary column.
2 ESTIMATION OF THE MATURITY OF A GAS:
PRIMARY VERSUS SECONDARY CRACKING.
POSSIBLE SECONDARY BIODEGRADATION
As the proportions and isotope ratios of methane are highly
subjected to post-genetic processes, a new attempt for
characterizing the maturity of the gas was done using the
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20
1000
Bacterial
10
Biodegradation
Field A
Field B
Field C
15
Maturity trend
Field D
Field E
10
Maturity
1
5
Thermogenic
0
-75
-65
-55
-45
0
-35
0
50
δ13C1
100
C2/iC4 (mol/mol)
150
Figure 4
Distinction of thermogenic and bacterial origin of natural gas,
combining the use of carbon isotopes and noble gases. The
gases presented in this diagram have unambiguous bacterial
or thermogenic origins, without any mixing or post-genetic
fractionation. Bacterial gases have light carbon isotopic
signatures, and high apparent production ratios 4He/40Ar*
compared to the average crustal value. Data from Prinzhofer
et al. (2000c) and unpublished data.
Figure 5
Diagram C2/C3 versus C2/iC4 (Prinzhofer et al., 2000b),
distinguishing a maturity trend from a biodegradation trend.
Some natural gas examples are shown exhibiting both
processes. Field A, gases have a lower maturity than field B,
both exhibiting a trend of biodegradation respectively.
Gas fields with primary cracking
Gas fields with secondary cracking
Primary
cracking
Primary
cracking
Secondary cracking of oil
0
Secondary cracking of oil
0
Secondary cracking
of oil and gas
Secondary cracking
of oil and gas
-1
-1
.5
-10
.5
δ13C2 - δ13C3
3
3
8
1.
8
1.
-2
-2
.0
.0
-15
-15
VR (%)
0
4
Secondary cracking
of gas
1.
1.
-10
.1
.1
Secondary cracking
of gas
-5
-1
-1
-5
0 .9
0 .9
δ13C2 - δ13C3
4He/40Ar*
100
Biodegradation
trends
C2/C3 (mol/mol)
Macuspana series 2
Macuspana series 1
Trinidad biodegraded reservoirs
Trinidad bacterial reservoir gas
Trinidad reservoir
Trinidad mud volcanoes
8
12
VR (%)
0
4
8
C2/C3 (mol/mol)
C2/C3 (mol/mol)
Ceara Basin
Thailand
12
Figure 6
δ13C2- δ13C3 versus C2/C3 diagram (Lorant et al., 1998), with two examples of gas series from the Ceara Basin, Brazil (Prinzhofer et al.,
2000b) and from Thailand. The first case presents a trend of maturity of an open system during the primary cracking of kerogen, the second
case presents a wider range of maturity in a more closed system, ranging from the primary cracking to secondary cracking of oil and gas.
A Prinzhofer and A Battani / Gas Isotopes Tracing: an Important Tool for Hydrocarbons Exploration
chemical and isotopic signatures of the C2-C4 fraction of
the gases (Lorant et al., 1998; Prinzhofer et al., 2000a;
Prinzhofer et al., 2000b). Two main diagrams were
established, the first one (C2/C3 versus C2/iC4) allowing to
distinguish any gas biodegradation versus a maturity trend.
Its principle is that for a maturity trend, iC4 proportions
decrease more rapidly than C3 proportions. On the contrary,
during a biodegradation process, propane (and nC4) are more
efficiently altered than iC4. Figure 5 gives some examples of
gas signatures plotted in this diagram. It is seen that for a
given basin, gases from some reservoirs exhibit a trend of
maturity, whereas for some other hydrocarbon accumulations, a trend of biodegradation may be defined.
An other diagram using both the chemical and isotopic
signatures of ethane and propane was developed in order to
better characterize the maturity of the gas. For correctly
interpreting data, an other important parameter to take into
account is the degree of openness of the system, as the
maturity trends depend drastically on the retention of the
hydrocarbons in the generation area or not. Figure 6 presents
gas data from two different basins, one corresponding to gas
generated during the primary cracking of the kerogen, the
other one where a large proportion of the gases comes from
the secondary cracking of the liquid hydrocarbons. This
efficiency of accumulation is primordial when characterizing
the composition of accumulated fluids. Indeed, as shown
in Figure 7, for the same source rock and for the same
maturation history, a reservoir accumulating all the produced
hydrocarbons will be filled with oil and gas, whereas a
reservoir accumulating only in the last part of the generated
hydrocarbons may be filled only with dry gas.
3 GAS AS A TRACER OF MIGRATION: STABLE
ISOTOPES VERSUS NOBLE GAS SIGNATURES
Segregation during the migration of natural gas may be
monitored through the carbon isotopes of methane
(Prinzhofer and Pernaton, 1997; Prinzhofer et al., 2000a). As
presented earlier, the carbon isotopes of methane fractionate
during migration with an enrichment in the light isotope 12C.
This observation obscures the diagnostic on the origins of
gases, but may help in the same time the reconstruction of the
directions of migration. However, as said before, these
signatures are strongly dependent on the kind of genesis of
the gas (bacterial versus thermogenic), on the isotopic
signature of the source and of its maturity, complexing the
fractionation due to gas migration through porous rocks.
On the contrary, because of their chemical inertia, isotopic
and chemical ratios of noble gases are fractionated only by
physical processes, i.e. in this case the interaction between
fluids and fractionation during diffusion. The ratios between
two fossil isotopes of noble gases (of two different chemical
species) are controlled by the atmospheric ratio, the relative
Principle of the efficiency of accumulation
for one source rock...
Fluid
generation
Early gas
cracking
Gas from oil
secondary cracking
N2
ge
ne
tio
n
Gas from
late primary
cracking
Oil
generation
ra
Bacterial
gas
Time
Case 1
Case 2
Accumulation
GOR: 200 wet gas
Accumulation
GOR: 3000 dry gas
Accumulation
Case 3
305
Dry gas
rich in N2
Figure 7
General scheme of hydrocarbon and N2 generation from organic matter. The range of accumulation time has a direct influence on the
composition of reservoired hydrocarbons, a late accumulation being enriched in gas as compared to a complete accumulation. For
overmature source rocks, a late accumulation may lead to a gas highly enriched in nitrogen.
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-0.2
10 km
Field C
Field A
-0.4
Latitude
-0.6
-0.8
-1.0
-1.2
North
-1.4
Eigenvector V2
-1.6
Field D
a)
-1.8
Longitude
Direction
of migration
10 km
Field C
Field A
3.3
3.1
Latitude
2.9
2.7
2.5
2.3
North
22Ne*/21Ne*
Field D
b)
2.1
1.9
Longitude
Figure 8
Migration maps in a series of reservoirs of a Brazilian basin (Prinzhofer et al., 2000d), using (a) the PCA method based on the proportion and
the carbon isotopic ratios of hydrocarbons (Prinzhofer et al., 2000a) and (b) the isotopic ratios of the nucleogenic part of 21Ne and 22Ne
(Prinzhofer et al., 2000d). Both tracers indicate a direction of migration from the southwest to the northeast.
solubilities of noble gas in water in the aquifer recharges, and
to fractionation processes due to migration in sedimentary
rocks. For radiogenic and nucleogenic isotopes, their relative
ratios are controlled by the average crustal production ratio
and by the transfer function from the rocks to the
hydrocarbons. It is then possible to characterize the direction
of migration with the same physical principle, which implies
that a light molecule moves faster than a heavy one. The
fractionation may occur for both fossil and radiogenic
isotopes, indicating a late fractionation. On the contrary, if
only the fossil isotopes are fractionated, this means that the
differentiation occurred before the integration of radiogenic
isotopes into the hydrocarbons. In Figure 8 are presented two
maps of fractionation due to migration. Figure 8a represents a
fractionation linked mainly to the carbon isotopic ratios of
methane, quantified with the second eigenvector of a PCA,
mainly linked with the carbon isotope composition of
methane (Prinzhofer et al., 2000a). An enrichment in 12C
from the southwest to the northeast indicates that the pod of
generation is likely located southwest of the reservoirs. When
comparing such results with a ratio of two nucleogenic
isotopes 21Ne* and 22Ne* (Fig. 8b), the same relative
enrichment in light isotopes is visible to the northeast of the
studied area, confirming the direction of accumulation of
hydrocarbons.
In other cases, mixture with bacterial methane will
obscure the fractionation of carbon isotopes due to migration,
while noble gases will not be affected by this hydrocarbon
mixing process, and will give a consistent direction of
migration. Figure 9 represents in the basin of Macuspana
(Mexico) the fractionation of ratios of fossil and radiogenic
isotopes respectively (Prinzhofer et al., 2000c). A consistent
direction of migration from south to north is visible, although
the values of δ13C of methane do not give a clear clue about
migration because of an important mixing between a bacterial
and a thermogenic end-member (Prinzhofer et al., 2000c).
Figure 9 c and 9d show that the projection of the iso- δ13C of
methane and ethane are similar, although some shift does
A Prinzhofer and A Battani / Gas Isotopes Tracing: an Important Tool for Hydrocarbons Exploration
Map of iso - 20Ne/36Ar
3.9
3.2
Mi
Mi
1.2
0.6 0.5
0.6
1.3
n
ratio
0.6
n
ratio
1.0
0.7
3.7
2.5 1.4
Mi g
g ra
tio
n
2.4
1.5 4.8
Mi g
g ra
tio
n
4.8
Map of iso - 4He/40Ar*
0.8 0.4
0.5
0.8
0.5
1.3
10 km
a)
b)
Map of iso - δ13C1
66
65
Map of iso - δ13C2
68
41
55
65
64
41
41 40
70
68
64
61
46
44
29
62
31
Thermo
62
36
58
59
41
36
Bio
c)
d)
Figure 9
Maps of several gas geochemical parameters in the Macuspana Basin, Mexico (Prinzhofer et al., 2000c).
a) map of the iso-values of the ratio of two fossil noble gas isotopes 20Ne/36Ar.
b) map of the iso-values of the ratio of two radiogenic noble gas isotopes 4He/40Ar*.
c) map of the iso-values of the carbon isotopic ratio of methane.
d) map of the iso-values of the carbon isotopic ratio of ethane.
The four maps indicate a consistent direction of migration from the south to the north.
307
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Oil & Gas Science and Technology – Rev. IFP, Vol. 58 (2003), No. 2
persist between the positions of the less negative δ13C. These
images were interpreted (Prinzhofer et al., 2000c) as due to a
mixing between a bacterial gas impregnated with a small
amount of thermogenic gas, whose proportion increases from
the edge to the center of the studied area. The shift between
the two maximum of δ13C between methane and ethane is
further interpreted as due to a higher fractionation of methane
because of longer migration. As the heaviest methane is
shifted south of the studied area, this would indicate a heavier
residue of leakage located south, i.e. a migration from the
south to the north, in perfect agreement with the fractionations of the noble gas isotopes. The combined approach
of δ13C and noble gas demonstrates the importance of gain of
information from all these gaseous species.
4 NEW CHALLENGES: RESIDENCE TIME
OF HYDROCARBONS, QUANTIFICATION
OF LEAKAGE OUT OF THE RESERVOIRS
As was assessed in the introduction, noble gases (He, Ne, Ar,
Kr, Xe) contain isotopes generated through natural nuclear
reactions occurring in the minerals. Some isotopes are
radiogenic, i.e. generated by the radioactive decay of a
radioactive isotope. The others are called nucleogenic, and
their generation occurs through nuclear reactions as (α,n) or
(n,α) on nuclei of Li, Mg, F, O, etc. The absolute amount of
generated stable radiogenic and nucleogenic isotopes is
increasing linearly with time, for compatible time ranges
involved in petroleum systems (several hundred of million
years). For a given radiogenic or nucleogenic isotope, a part
of it comes from generation occurring before the history
of the petroleum system, and presents an atmospheric
proportion when compared to the same isotopes of the same
elements. This makes possible the distinction, for a given
concentration of a so-called radiogenic or nucleogenic
isotope, of a “fossil” part and of a “radiogenic” part, if an
other isotope of the same element is only fossil. Taking the
example of argon, as 36Ar is only fossil, it is possible to
calculate the concentration of fossil 40Ar (40Aro), knowing the
atmospheric ratio 40Ar/36Ar equal to 295.5:
40Ar
o
= 295.5 · 36Ar
(1)
and the concentration of the radiogenic 40Ar (40Ar*):
40Ar*
= 40Ar – 40Aro = 40Ar (1-295.5 · 36Ar)
(2)
As both isotopes, fossil and radiogenic/nucleogenic, are
rigorously the same physical element (i.e. will not suffer any
physical or chemical fractionation as they are physically
undistinguishable), the ratio of radiogenic (or nucleogenic)
over fossil concentration is linearly linked to the residence
time of the hydrocarbon fluid. The coefficient of
proportionality is not constrained yet, but for a given
geological situation, as for example, several fields that
produce with the same geological interval and with
hydrocarbons originating from the same source rocks, it is
possible to quantify in a relative way the residence times of
hydrocarbon accumulations. Figure 10a shows that for the
same example than illustrated and discussed on Figure 8, the
gases from the two fields A and B have similar ratios in a
diagram 40Ar* versus 40Aro, and their different values define
a straight line passing through the origin. The gases from
fields C and D define an other straight line with a slope
presenting a larger value. The ratio between the two slopes
equals 1.34. Looking at other nucleogenic isotopes as 21Ne
and 22Ne for the same gas wells, the same kind of diagram is
visible, with ratios between the two linear correlation trends
of 1.30 and 1.44 respectively (Figs 10b and c). The good
consistency between these three values of slope ratios
confirms the quantitative estimate of the relative residence
time of hydrocarbons in these reservoirs, the hydrocarbons of
reservoirs C and D having a residence time approximately
35% longer than in the reservoirs A and B.
Another new powerful application of noble gas isotopes
consists on using them as inert tracers of leakage from the
reservoirs through caprocks or aquifers. In fact, it is
common sense to say that helium leaks much more easily
than other gases because of its size (except hydrogen). As
the noble gas family corresponds to a series of gas
molecules from helium to xenon with increasing size, the
latest being a large molecule which does not migrates easily,
it is possible to trace a leakage phenomenon looking either
at concentrations of different noble gas compounds, or at
ratios between two noble gas compounds of different size,
the smaller moving out more rapidly than the larger. An
increase of a “leaking” process should result in a decrease of
the helium concentrations, and an increase of the ratios
(heavy compound/light compound). Figure 11 shows for the
same four fields of Figures 8 and 10, some correlation
diagrams, plotting a ratio of two fossil noble gas isotopes
versus the concentration of 4He. As the concentrations of
4He decrease, all the ratios heavy/light compounds increase.
The two trends for each diagram correspond to the two
families of fields, A-B and C-D respectively. It has been
noticed that the absolute concentrations of argon are
relatively constant in these families, whereas the concentrations of helium and neon are decreasing versus the
importance of accumulation leakage, while the concentrations of krypton and xenon are increasing. This indicates
that the leakage factor for the hydrocarbons is in the same
order than the leakage of argon, as the argon concentrations
— constant—correspond to the ratios of the absolute
amount of argon over the whole gas amount. It is then
possible to quantify the proportions of hydrocarbons lost
through leakage for each gas sample, fitting the curves of
Figure 11 with different diffusivities for helium, neon,
argon, krypton and xenon (for the noble gas ratios), and
using the same diffusivity for argon and for hydrocarbons
309
A Prinzhofer and A Battani / Gas Isotopes Tracing: an Important Tool for Hydrocarbons Exploration
12
40Ar*
36Ar/20Ne
4E-04
2E-04
8
4
Slope ratios = 1.34
0
0
0
2E-05
4E-05
40Ar
a)
0
6E-05
0.001
0.002
0.003
4He
a)
o
1.2E-10
0.5
0.4
21Ne*
84Kr/20Ne
8E-11
4E-11
0.3
0.2
0.1
Slope ratios = 1.30
0
0
5E-11
1E-10
21Ne
b)
0
1.5E-10
0
0.002
0.001
b)
o
0.003
4He
0.016
3E-10
0.012
22Ne*
136Xe/20Ne
2E-10
1E-10
0.008
0.004
Slope ratios = 1.44
0
0
0
2E-09
22Ne
c)
Field A
Field B
4E-09
0.001
Field D
0.002
0.003
4He
c)
o
Field C
0
Field A
Field B
Field C
Field D
Figure 11
Figure 10
Correlation between the fossil and the radiogenic/nucleogenic
part of the isotopes 40Ar, 21Ne and 22Ne for the four fields of
Figure 8. The ratios X*/Xo are proportional to the residence
time of the hydrocarbons.
Correlations between the ratios of two fossil isotopes of noble
gases and the concentrations of the radiogenic 4He for the
four fields from Figure 8. The observed trends are interpreted
as due to leakage, which decreases the 4He concentrations
and increases the ratios heavy/light isotopes.
310
Oil & Gas Science and Technology – Rev. IFP, Vol. 58 (2003), No. 2
for the helium concentrations. The result of this calculation is
presented in the map of Figure 12. The proportions of
displaced hydrocarbons range between 16 and 80%, and their
repartition indicates that fields B and D exhibit smaller
leakage than fields A and C.
Field A
70
60
50
Field B
40
North
30
Field D
Battani, A. (1999) Utilisation des gaz rares He, Ne et Ar pour
l’exploration pétrolière et gazière : exemple des accumulations du
Pakistan et de Macuspana (Mexique). PhD, Paris University.
Battani, A., Sarda, Ph. and Prinzhofer, A. (2000) Geochemical
Study of Pakistani Natural Gas Accumulations Combining Major
Elements and Rare Gas Tracing. Earth and Planet. Sci. Lett., 181,
229-249.
Leakage (%)
Field C
Ballentine, C.J. and Sherwood-Lollar, B.S. (2002) Regional
Groundwater Focusing of Nitrogen and Noble Gases into the
Hufoton-Panhandle Giant Field, USA. Geochim. Cosmochim.
Acta, 66, 14, 2483-2497.
20
10 km
Figure 12
Map of the quantified proportion of hydrocarbons lost by
leakage for the four fields of Figure 8.
CONCLUSIONS
From the recent developments in the geochemistry of natural
gases, it appears that the use of carbon isotopes, extended to
the whole range of gas molecules, gives an evaluation of the
physico-chemical processes affecting hydrocarbons in
sedimentary basins, including the source of the hydrocarbons,
their genesis and their post-genetic history as their migration,
accumulation, leakage and chemical alteration (bacterial
and/or thermal). The recent development of noble gas
isotopes tracing of these fluids gives a more accurate view of
some of these processes, as the type of genesis and the
secondary and tertiary migration of hydrocarbons. Some very
new quantitative informations about the proportions of
displaced hydrocarbons, and the residence time of hydrocarbons in reservoirs is still under development, giving to gas
geochemistry new challenges for future applications outside
oil and gas exploration, as for example the monitoring of
hydrocarbon production, the monitoring of CO2 sequestration
in aquifers or in depleted hydrocarbon reservoirs.
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Final manuscript received in February 2003